1. Field of the Invention
The present invention generally relates to valves, and more particularly to full sealing throttle valves.
2. Description of Related Art
Butterfly valves are commonly used in industry to throttle flow rates in a flow channel by adjusting the position of a throttle plate within the channel. By rotating the throttle plate from a position parallel to the flow to a position perpendicular to the flow, the flow rate decreases. Positioned perpendicularly, however, the throttle plate by itself only impedes the flow, but does not stop it.
Because of the typical construction of butterfly valves, sufficient clearance must exist in the valve bore to permit unobstructed movement of the throttle plate in the bore. The necessary clearance, however, prevents the plate from fulling blocking the bore.
In many applications it is desirably to regulate and cease the flow through the flow channel. Previously, butterfly valves included an O-ring on the throttle plate periphery edge which compressed between the flow channel wall and the throttle plate periphery edge when closed. Although these designs produce somewhat effective seals, a control problem manifests because O-rings typically release pressure asymmetrically (i.e., release pressure on the leading edge of the throttle plate first) causing pressure spikes across the valve.
To overcome this problem, butterfly valve designs developed which rotate the throttle plate into the perpendicular position (i.e., a close position) and then axially translate the throttle plate to seat against a seal (i.e., translate into a seal position). Examples of butterfly valves with axial translation into a seal position are disclosed in U.S. Pat. Nos. 4,817,916 and 4,921,212.
U.S. Pat. No. 4,817,916 to Rawstron discloses a butterfly valve axially actuated by a cam mounted on the valve shaft. The cam cooperates with a cam chamber surface to rotate the throttle plate into a close position. In the close position, the cam rotates across the cam chamber and engages a second cam chamber surface. Continued rotation of the shaft with the cam engaging the second surface forces the throttle plate linearly into a seated position. The '212 patent functions in a similar manner.
Although the butterfly valves of these references disclose axially displacing the throttle plate to seal the plate, all of the designs produce additionally control problems because the control of throttle plate movement by the actuator shaft does not extend continuously over the entire range of shaft movement. In other words, the axial translation of the throttle plate does not directly correspond to rotation of the actuator shaft from the close position to the seal position.
The prior art devices are therefore susceptible to mechanical hysteresis because the throttle plate does not directly communicate with the shaft, i.e., during a portion of shaft rotation, the plate does not respond. Mechanical hysteresis produces control problems which lead to system instability.
Moreover, the prior art discloses massive throttle plate assemblies with inherently slow response time. As known in the art, the larger the mass of rotation, the slower the response time. In semiconductor processing, valve response time is essential in a vacuum system. Consequently, the prior art devices are not suited for such application.
Thus a need exists for a full sealing butterfly valve in which the axial translation of the throttle plate directly corresponds to the rotation of the actuator shaft to improve control of the valve over the entire range of throttle plate movement from a full open position to a seal position. In addition, a need exists for a butterfly valve having a quick response time.